There are two types of processes for the manufacture of polyethylene that involve the polymerization of monomers in an inert liquid medium in the presence of a coordination catalyst viz. those which operate at temperatures below the melting or solubilization temperature of the polymer and those which operate at temperatures above the melting or solubilization temperature of the polymer. The latter are referred to as "solution" processes, an example of which is described in Canadian Patent 660,869 of W. W. Anderson, E. L. Fallwell and J. M. Bruce, which issued Apr. 9, 1963. In a solution process, the process is operated so that both the monomer and polymer are soluble in the reaction medium. Accurate control over the degree of polymerization, and hence the molecular weight of the polymer obtained, may be achieved by control of the reaction temperature. In solution polymerization processes, it is advantageous to operate the process at very high temperatures e.g.&gt;250.degree. C., and to use the heat of polymerization to flash off solvent from the polymer solution obtained.
While steps may be taken to remove catalyst from the polymer subsequent to the polymerization step in the process, it is preferred that a solution polymerization process be operated without catalyst removal step. Thus, catalyst will remain in the polymer. Such catalyst, which may be referred to as "catalyst residue", may contribute to the colour of the polymer obtained and to degradation of the polymer during or subsequent to processing of the polymer. The amount of catalyst residue is related, at least in part, to the overall activity of the catalyst employed in the polymerization step of the process as the higher the overall activity of the catalyst the less catalyst that is, in general, required to effect polymerization at an acceptable rate. Catalysts of relatively high overall activity are therefore preferred in solution polymerization processes.
Two important factors in determining the overall activity of a catalyst are the instantaneous activity of the catalyst and the stability of the catalyst under the operating conditions, especially at the operating temperature. Many catalysts that are stated to be very active in low temperature polymerization processes also exhibit high instantaneous activity at the higher temperatures used in solution processes, but tend to decompose within a very short time in a solution process, with the result that the overall activity is disappointingly low. Such catalysts are of no commercial interest for solution processes. Other catalysts may exhibit acceptable overall activity at the higher temperatures of a solution process but show tendencies to yield polymers of broad molecular weight distribution or of too low a molecular weight to be commercially useful for the manufacture of a wide range of useful products. Thus, the requirements for and the performance of a catalyst in a solution polymerization process are quite different from those of a catalyst in a low temperature polymerization process, as will be understood by those skilled in the art.
The preparation of polymers of ethylene in solution polymerization processes is described in published PCT patent application No. WO 91/17193 of D. J. Gillis, M. C. Hughson and V. G. Zboril, published Nov. 14, 1991 and in the patent applications referred to therein. Catalysts activated by siloxalanes are capable of polymerizing ethylene at very high temperatures. However, the siloxalane residues from such catalysts tend to significantly adversely affect the performance or adsorbers used to purify solvent in the associated solvent recovery and recycle sections of the polymerization process.
There is extensive prior art on the use of various electron donors as adjuncts to Ziegler-Natta catalysts in low (less than 90.degree. C.) temperature polymerization of ethylene and other alpha-olefins, to increase the activity and/or stereospecificity of the catalyst. Esters of aromatic acids e.g. toluic or benzoic acid, ethers and alcohols are frequently used for that purpose. However, most electron donors that are useful at low temperatures destroy catalyst activity as the polymerization temperature increases. As an example of the use of electron donors, U.S. Pat. No. 4,097,659 of H. M. J. C. Creemers et al., issued Jun. 27, 1978 discloses a low temperature polymerization process, operating in an inert solvent at temperatures in the range of 20.degree.-100.degree. C., in which the list of examples of activators includes dimethylmonobutoxy aluminum, monodecylpropoxy aluminum chloride and monobutyl monobutoxy aluminum hydride.
As exemplified hereinafter, substitution of even part of trialkylaluminum with alkoxy alkylaluminum of the type used in U.S. Pat. No. 4,097,659 results in a substantial decrease in catalyst activity even if the temperature is only 130.degree. C. i.e. in the lowest temperature range of operation of a solution polymerization process. Surprisingly, it has now been found that at higher temperatures this trend to decreased catalytic activity is reversed and alkoxyalkyl aluminum activated catalysts exhibit superior activity at temperatures above about 180.degree. C.
European Patent application 0 280 353 assigned to Stamicarbon B.V. published Aug. 31, 1988 discloses a catalyst which comprises a very broad number of possibilities for forming a catalyst in terms of the various components in the catalyst. However, the patent application teaches a first component relatively rich in aluminum and chlorine. The preferred ratio of Cl/Mg is greater than 3, preferably greater than 5 (page 3 line 26) which is greater than the Cl/Mg ratio in the catalysts of the present invention. Further the Stamicarbon disclosure does not teach catalysts comprising a vanadium component which come within the scope of the present invention. Accordingly, the Stamicarbon reference teaches away from the subject matter of the present patent application.